Technical Abstract:
Aspergillus flavus is the most common species associated with agricultural aflatoxin contamination. A. flavus soil populations contain isolates from two morphologically distinct sclerotial size variants, termed the L-strain for isolates with average sclerotial size greater than 400 um and the S-strain for isolates with sclerotial size less than 400 um. On typical laboratory growth media, S-strain isolates produce higher levels of aflatoxins, more abundant sclerotia, and fewer conidia. Atoxigenic S-strain isolates are very rarely found in natural environments, whereas as much as 40 percent of the L-strain isolates can be atoxigenic. A. oryzae, used in some food fermentation processes, is a variant atoxigenic A. flavus with characteristics of either S or L strain isolates. A. flavus lacks the ability to produce G-aflatoxins due to a gap in the cluster that includes a required cytochrome P450-encoding gene, cypA. The size of the deletion that causes loss of a portion of cypA is 1.5 kb for S-strain isolates and 0.8 kb for L-strain isolates. Soil populations of A. flavus are typically composed of isolates from hundreds of different vegetative compatibility groups. Although frequent genetic exchange among these groups has not been observed, historical recombination in populations probably has occurred. Based on the phylogenetic relationship of A. flavus and A. oryzae, S-strain isolates and some A. oryzae isolates are most likely descended from an aflatoxin-producing L-strain common ancestor. We hypothesize that the frequent loss of aflatoxin-producing ability in A. flavus isolates from agricultural soil could be a consequence of adaptation to a carbon-rich environment that makes the aflatoxin cluster less genetically stable.